The patient with acute recurrent laryngeal nerve (RLN) injury suffers not only the functional consequences of impaired vocal fold motion, but also the psychological impact of reduced communication. Patients with unilateral RLN injury are variably disabled by aphonia and aspiration, while bilateral RLN injuries obstruct the airway, thereby necessitating tracheotomy. These conditions are often the direct result of inappropriate reinnervation of laryngeal muscles by axons that are misdirected in the course of regeneration. The process of reinnervation is influenced by surface molecules that promote cell adhesion. Ideally, this process should simulate the normal developmental process wherein axon-axon and axon- muscle connections are made correctly. When proper connections fail to develop, residual laryngeal weakness and impairment of function occur. The purpose of this study is to define the cellular changes in motor unit organization responsible for observed patterns of RLN reinnervation of the rat laryngeal musculature. The proposed study continues earlier work which suggests that patterns of laryngeal reinnervation are subject to preferential contact of the laryngeal muscles by adductor motoneurons of the nucleus ambiguus. To further evaluate the process of laryngeal reinnervation in the rat model, regenerating motoneurons will be classified according to their original function using retrograde labeling techniques. Cell number and somatic size will be correlated with metabolic alterations associated with nerve injury using histochemical assays for succinic dehydrogenase and cytochrome oxidase. Immunocytochemistry techniques will be employed to evaluate the role of cell adhesion molecules in the process of reinnervation, specifically addressing expression with respect to laryngeal motoneuron and muscle function. Patterns of cell adhesion molecule expression in repair will be compared to the events occurring in the normal developing central and peripheral nervous system. This CIDA application is based on the proposed studies and advanced training in developmental neurobiology, immunocytochemistry, electron microscopy and biostatistics. The basic training is intended to develop an applied research program focused on laryngeal development and mechanisms of RLN regeneration.